A zero-turn riding lawn mower (colloquially, a z-turn) is a standard riding lawn mower with a turning radius that is effectively zero. Different brands and models achieve this in different ways, but hydraulic speed control of each drive wheel is the most common method. Both commercial duty and homeowner models exist, with varying engine power options, size of cutting decks, fuel type (gasoline or diesel), and prices. A z-turn mower typically drives faster and costs more than a similarly sized conventional riding mower that has steerable front wheels. Most current models have four wheels: two small swiveling front tires and two large drive tires in the back. Bush Hog mowers sometimes come with a small, pivoting fifth wheel mounted in the center behind the driver. Instead of controlling the swiveling tires to steer the machine, the large drive tires rotate independently of each other based on the driver's input. They may rotate in opposite directions. The mower can pivot around a point midway between the drive wheels (the classic z-turn), or it can pivot around either one of the drive wheels if one is stationary, or it can turn in a circle of any radius. Reversal of the direction of travel can be accomplished by causing both wheels to rotate in reverse.